Methods for determining incident received power or energy making use of antenna systems are known, by means of which at least two or however also more asymmetric radiation patterns predeterminable in their direction and position and determined by the antenna geometry can be generated. The radiation patterns maybe conical or fan-shaped; they are also called radiation fan-beams. In the known method, the radiation patterns may mutually penetrate each other in predeterminable manner in intersection zones and numbers and with arbitrary spatial distribution.
The expression "intersection zone" is defined as the set of those radiation patterns or fan beams of which the mutual penetration is determined mainly but not necessarily solely in a given main direction of radiation. In this manner and in the known method, it is possible to synthetically generate narrower radiation patterns, namely using so-called unfilled apertures (see "Interferometry and Synthesis in Radioastronomy", A. Richard Thompson, James M. Moran, George W. Swenson, pp 118, Krieger Publishing Company, 1991; "The Synthesis of large Radio Telescopes", M. Ryle, A. Hewish, in Instrumentation and Techniques for Radio Astronomy, p 97) are also mainly characterized in that they comprise several single sensors, for instance antennas, the conical or fan-beam radiation patterns being formed by known beam-shaping means and methods from the output signals of said single sensors. Accordingly when using unfilled apertures, it is possible to generate simultaneously from the single sensors' output signals an arbitrarily, predeterminable number of radiation patterns.
An example of a beam-shaping means generating simultaneously a given number of directionally determined radiation patterns (fan beams) from the output signals of a row of single sensors is the Butler matrix (see for instance "Antenna Engineering Handbook", Richard C. Johnson, chapter 20-56, McGraw-Hill Inc. 1991) which uses analog means to reproduce the function of a Fourier transformation (FFT). The radiation patterns, for instance fan beams, however also may be generated using different radiation shaping means (see for instance "Computational Methods of Signal Recovery and Recognition", Richard J. Mammone, pp 272, John Wiley & Sons Inc., 1992), for instance by using a phase network.
The unfilled apertures also comprise those making it possible that different fan-beam or conical radiation patterns mutually penetrate in localized pattern zones. This is the case for instance for rows of antennas subtending an angle with each other. One the many possible ways is the so-called Mills Cross wherein two rows of dipoles are mutually orthogonal, for instance in cross or L shape. Such arrays of unfilled-aperture antennas and sensors allow generating asymmetric, fan-beam or conical radiation patterns which are considerably narrower in one plane than in another.
Radiation patterns which are rotationally symmetric or which are approximately of equal widths in all planes can be generated directly only using filled apertures, however entailing complexity and costs which are a multiple of those of unfilled apertures. An example relating to a filled aperture is the surface of a parabolic mirror. An example relating to an unfilled aperture is a row of dipoles.
In the known methods, the fan beams forming an intersection zone are processed by analog or digital apparatus to form a new radiation pattern by correlation. Because a signal arriving from the intersection point or intersection zone simultaneously is a common element of the intersecting radiation patterns, a correlation result is present at the intersection point in the form of an antenna lobe which on the whole is narrower and more sharply defined than the fan beams or radiation patterns forming the intersection zone.
If for instance in the known methods a Mills Cross is formed that consists of two rows of dipoles, then, using the aforementioned Butler matrix, it is possible to generate a predetermined number of mutually orthogonal fan beams which can be mutually correlated in all desired combinations in their intersection zones.
The known method incurs a drawback in that other sources of interference or signals not arising in the intersection zone but incident on a fan beam will spuriously affect the measurement taken in the direction of the intersection zone, independently of the synthetic formation of the pattern. In particular as regards simultaneous spatial distribution of a full set of the intersection zones formed by the fan beams, unambiguousness will be entirely lost in the presence of interference.
In the known methods, a radiation pattern is formed from correlation and this pattern ideally will incorporate only signals coming from the direction of the common intersection zone. However the correlation differs from those procedures which vectorially combine the signal voltages of the single sensors, for instance in an analog network, in phase and amplitude. A large noise source present in the fan beam pattern but not incident along the main radiation direction will produce, without being regularly reduced by the correlation, a corresponding received power in the analyzer of the pertinent fan beam. As a result, a real signal incident in the main direction of radiation must be processed in the affected fan-beam channel at a higher receiver noise level than in the absence of said noise source. The actual signal in the presence of noise is hidden deeper in noise than without said noise source, the correlations will be different in each case. The difference in correlation results does not change the shape of the correlation function or that of the synthetic radiation patterns, but only the value of the correlation.
Illustratively if it is assumed that the noise source is a pulsed source and that the real signal also is formed in the main direction of radiation by a pulsed noise signal, then correlation no longer can unambiguously distinguish between pulse timing and its associated source in the known method. If the interference source of the one main direction of radiation simultaneously is the real signal in another direction of a common fan beam, then the signals no longer can be unambiguously distinguished by correlation in the known method. Consequently the received power or energy in turn no longer can be unambiguously ascertained along a main direction of radiation. In apparatus comprising a substantial number of mutually intersecting fan beams, the unambiguousness of the measurement will be completely lost.
Accordingly another drawback of the known method is that only a single signal source may be present within a given frequency bandwidth or that the signal level at least must be large compared to other signals (see "Radio Astronomy", John D. Kraus, chapter 6-25, Cygnus Quasar Books, 1986).
This feature entails another drawback of the known method using sensors having unfilled apertures, namely that it is impossible to simultaneously and unambiguously determine the received power of signals only differing by the direction of incidence within a given frequency bandwidth.